The present invention relates to a heavy-duty pneumatic radial tire wherein the first belt adjacent to a carcass layer has a split structure. More particularly, the present invention is concerned with a heavy-duty pneumatic radial tire wherein a drawback of a split structure, i.e., the problem of an uneven wear occurring at the central portion of a tread has been eliminated while maintaining excellent off road durability derived from the split structure of the first belt.
In general, heavy-duty pneumatic radial tires for trucks, buses, etc. have a belt structure comprising three belts each mad of metallic cords provided between a tread and a carcass layer. In this belt structure, the first belt adjacent to the carcass layer has a cord angle of about 40.degree. to 75.degree. relative to the circumferential direction of the tire and serves as a belt-reinforcing layer, while the second and third belts each have a cord angle of about 10.degree. to 30.degree. relative to the circumferential direction of the tire and cross each other to serve as a tension-resistant layer.
In the above-described belt structure, the first belt is disposed on substantially the whole part of the crown portion to reinforce the tire so as to cope with the pneumatic pressure, thereby ensuring the dimensional stability of the tire. Further, the above-described belt structure contributes to an enhancement of the cross-sectional flexural rigidity in the radial direction of the tire (i.e., the widthwise direction of the tire) over the whole ground-contacting area of the tread, which brings about an excellent effect of improving the uneven wear resistance and driving stability. However, this belt structure cannot follow the movement of the tread surface in response to unevennesses of the ground such as stones and projections, so that there occurs the concentration of stress in response to these unevennesses. This disadvantageously causes not only the tread at the central portion of the crown to be damaged but also the cords of the belt layer within the tire to be broken.
In order to eliminate the above-described disadvantages, a proposal was made on the so-called "split structure" formed by splitting the above-described first belt into two portions and disposing the two split portions respectively on both shoulder portions to form a space in the central region of the crown portion. In this proposal, the split structure was formed in the first belt to lower the cross-sectional flexural rigidity of the central region of the crown portion in the radial direction of the tire to thereby relax the stress.
Although a tire having the above-described split structure exhibited the above-described effect when travelled on a rough road having unevennesses, it brought about another problem when travelled at a high speed on ordinary pavement. Specifically, since the belt-reinforcing action in the central region of the crown portion was weakened, an increase in the growth of the central region of the crown towards the circumferential direction thereof was induced by inflation force of the internal pressure, which rendered the configuration of the crown portion unstable. This brought about a gradual increase in the strain occurring between the tension-resistant layers, i.e., the second and third belts, and finally the occurrence of the separation at the end portions of the belts. Further, this unfavorably brought about a lowering in the driving stability under heavy load conditions.
Accordingly, the present inventors have made extensive and intensive studies with a view to eliminating the above-described problems and, as a result, have found that the belt durability under high-speed travelling conditions can be ensured by disposing an organic fiber cord layer provided at a cord angle of 0.degree. to 10.degree. relative to the circumferential direction of the tire in the space portion of the above-described split structure in the first belt, or disposing two inextensible fiber cord layers each having a specific tensile modulus and crossing each other at a specific angle on the above-described space portion.
However, radial tires having the above-described belt structure brings about a problem of the so-called "uneven wear of center tread" severer than that in the case of the conventional tire provided with a first belt having no split structure, i.e., causes the tread wear rate in the central region of the crown portion to be higher than that in both shoulder regions. In particular, this uneven or irregular wear tends to occur when the tire is mounted on a driving axle. This brought about a problem that the tire should be replaced without accomplishing the intrinsic service life (travel distance) of the tire.